Obviously a response to market conditions. While there are several relevant ones here, including overly-aggressive pricing strategy in the last few years, camera/tablet cameras, mirrorless systems, a still-sluggish economic recovery, market saturation, etc., I can think of no stronger operative market force than competition.
And because Canon is partly insulated from Nikon/Sony competition by the investment its existing customers have sunk in Canon equipment (which acts as a barrier to switching systems), I think this speaks volumes about the effect of 3rd party lens makers in particular, being direct competition on lenses. Chief among them being Sigma, with their generally well-respected Art line.
I think this is great for the consumer, as it should drive innovation as well as put some brake on pricing.